Rigify - How do I change 'Bend Rotation Axis' parameter using Python?
Figured it out:

 bpy.data.objects['metarig'].pose.bones['thumb.01.L'].rigify_parameters.roll_alignment = 'manual'
 bpy.data.objects['metarig'].pose.bones['thumb.01.L'].rigify_parameters.copy_rotation_axes = [True, False, False]